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Old 09-21-2013, 08:50 PM   #9 (permalink)
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Location: Oxford, UK
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Black Beast - '02 VW Goff Estate S
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Originally Posted by Ryland View Post
Ebay has, for $90, 30amp, 36v to 12v DC to DC converters, you would still want a $25 ridding lawn mower starting battery to turn the starter over, but 3 cheap deep cycle batteries would give you alternator-less driving for 100's of miles without dimmed lights.
Sounds good. Do you have any details of the dc/dc converters?

I was thinking of using a couple of 150w car laptop adapters from Maplins set to 15v regulated output. A similar price to achieve a similar output. (Well, 250 watts seems about right to cover the load during normal running conditions.) Laptop adapters tend to be fairly efficient these days. They take a nominal 12 volt input but I have been told they will accept anything between about 11 volts and 15 volts input. In other words, they are designed to run on lead acid batteries, even significantly discharged ones. Using two such adapters in parallel should work fine I think, especially if they are feeding a system which has a big capacitor attached, in the form of the regular starter battery.

My hope is that by feeding in a sufficient current at a voltage just above the voltage at which the alternator regulates its output (14.4v?) the alternator will be 'fooled' into 'thinking' that the car's electrical load is zero watts and will therefore spin easily around without putting any power into the car. (15 volts is probably just about right, and happens to be a common laptop PSU voltage.) When peaks of current are demanded, the dc/dc converters (laptop adapters) will not be able to provide that much current, so the peak current will be provided by the starter battery, voltage will drop below 14.4v, and the alternator will respond by kicking out amps as normal, ...but for most of the time the alternator will be idle.

Someone, please, correct me if I'm wrong here but I think this would mean that no switches or alterations would be necessary at all, beyond feeding that regulated 15v output into the car's electrical system at some suitable point. When the auxiliary battery becomes exhausted and its voltage drops, the adapaters will no longer be able to maintain the 15v and the aux battery should stop discharging. The alternator will then take over.

Ideally the battery should be about 250Ah. Perhaps 2x 6v 250Ah batteries in series, so each battery can be moved around without lifting gear! But I'll start with one cheap 12v 65Ah leisure battery to test the concept. That will be light enough to carry indoors every evening to put on charge overnight.

15 volts is slightly higher than the normal voltage in my car but should not cause a problem, either for the battery, or the ECU, or for any other component in the car. Indeed, some alternators will output 15 volts anyway. 15.5v might be too high but 15 volts should be no problem.

Someone correct me if my logic is wrong here! What have I missed?

Last edited by paulgato; 09-22-2013 at 03:19 AM.. Reason: Typo - changed 'with' to 'without'.
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